linux-kernel - Re: [PATCH v8] arm64: cpuinfo: Expose MIDR_EL1 and REVIDR

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Date:	Tue, 5 Jul 2016 15:56:31 +0100
From:	Will Deacon <will.deacon@....com>
To:	Suzuki K Poulose <suzuki.poulose@....com>
Cc:	catalin.marinas@....com, mark.rutland@....com,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	steve.capper@...aro.org, stuart.monteith@...aro.org
Subject: Re: [PATCH v8] arm64: cpuinfo: Expose MIDR_EL1 and REVIDR_EL1 to
 sysfs

On Mon, Jul 04, 2016 at 10:20:20AM +0100, Suzuki K Poulose wrote:
> From: Steve Capper <steve.capper@...aro.org>
> 
> It can be useful for JIT software to be aware of MIDR_EL1 and
> REVIDR_EL1 to ascertain the presence of any core errata that could
> affect code generation.
> 
> This patch exposes these registers through sysfs:
> 
> /sys/devices/system/cpu/cpu$ID/regs/identification/midr_el1
> /sys/devices/system/cpu/cpu$ID/regs/identification/revidr_el1
> 
> where $ID is the cpu number. For big.LITTLE systems, one can have a
> mixture of cores (e.g. Cortex A53 and Cortex A57), thus all CPUs need
> to be enumerated.
> 
> If the kernel does not have valid information to populate these entries
> with, an empty string is returned to userspace.

[...]

> +static int cpuid_add_regs(int cpu)
> +{
> +	int rc;
> +	struct device *dev;
> +	struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
> +
> +	dev = get_cpu_device(cpu);
> +	if (dev) {
> +		rc = kobject_add(&info->kobj, &dev->kobj, "regs");
> +		if (!rc)
> +			rc = sysfs_create_group(&info->kobj, &cpuregs_attr_group);

If this call fails...

> +	} else {
> +		return -ENODEV;
> +	}
> +
> +	return rc;
> +}
> +
> +static int cpuid_remove_regs(int cpu)
> +{
> +	int rc = 0;
> +	struct device *dev;
> +	struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
> +
> +	dev = get_cpu_device(cpu);
> +	if (dev) {
> +		sysfs_remove_group(&info->kobj, &cpuregs_attr_group);

... then we still call sysfs_remove_group on the CPU_DEAD path. I think
that just results in a WARN, but it would be good to double-check this
(perhaps by forcing the failure path).

Will